M.L.
Let her dress up as a boy if that's what she wants! You might be able to compromise on the hair and ask if a ponytail would be okay or let it be long like Tarzan? It's okay for her to be interested in "boy" activities and if she's usually had "girl" toys around, maybe she's just exploring things that are new to her!
If there are times when it really matters what she wears (such as Easter, a family gathering, etc.) then maybe mark it on the calendar and let her know that you are chosing what she wears for the event that day. She can see that it's coming and prepare herself. She might even be willing to do a "6 days you pick, 1 day I pick" thing. You could give her stickers to put on the calendar for the days she chooses what she wears to help her see it. She's probably too young to reason really, but she can understand and visual aides are probably right up her alley!